1. Field of the Invention
The present invention relates to a high-strength steel plate with excellent electromagnetic shield and hot-dip galvanization properties, which is suitable for use where high corrosion resistance is required, like construction finishes. More particularly, the present invention relates to a high-strength steel plate with excellent electromagnetic shield and hot-dip galvanization properties, which shows a shield effect of 25 dB (shield efficiency 93%) or higher against electromagnetic fields at 60 Hz and a yield strength of 22 kg/mm2 or higher.
2. Description of the Prior Art
Many natural and human-made sources generate electromagnetic energy in the form of electromagnetic waves. These waves consist of oscillating electric and magnetic fields which interact differently with biological systems such as cells, plants, animals, or human beings. The finding of electromagnetic waves having detrimental effects on the body has led to the development of various methods and materials for shielding electromagnetic waves. Waves adversely affecting the body are collectively called harmful waves.
Recent studies have demonstrated harmful effects of electromagnetic waves at low frequencies on biological systems. Particularly, a series of studies results revealing the interrelation of the electromagnetic field (60 Hz) around power transmission lines with carcinogenesis has had great repercussions all over the world.
In addition to carcinogenic effects, low frequency waves with magnetic properties are found to cause inductive currents in the body upon exposure to the waves for a long period of time, upsetting the biological balance of various ions, such as Na+, K+, Cl− and so forth, across cellular membranes, which results in adversely affecting the hormone secretion and immunocytes of the body.
Further, other studies showed the influence of magnetic fields on the secretion of melatonin, a hormone responsible for regulating the sleep cycle, adding that the body may suffer from insomnia upon prolonged exposure to magnetic fields.
In order to cope with such harmful electromagnetic waves, shielding technology has been developed in two aspects: structure and material. With regard to the construction aspect, magnetically shielded rooms are disclosed in U.S. Pat. No. 6,282,848 and Japanese Pat. Laid-Open Publication No. Hei. 7-32136. Electrically conductive materials such as copper are in current use as shields against electromagnetic waves, as disclosed in Japanese Pat. Laid-Open Publication No. 2001-217589. However, such materials are useful only for electromagnetic waves at high frequencies (1 KHz or higher).
Electromagnetic waves at 60 Hz, usually detected in general power sources, are composed of an electric field and a magnetic field component, which both vary with time. Accordingly, in order to shield these low frequency electromagnetic waves, which have recently been shown to have adverse health effects, time-varying electric and magnetic fields should be considered together. However, there have not yet been developed practical technologies for steel plates that can effectively shield time-varying electromagnetic fields.
Conventionally, steel plates with high magnetic permeability are used as magnetic shields. For instance, Japanese Pat. Laid-Open Publication Nos. Hei. 10-208670 and Hei. 10-96067 and PCT WO 97/11204 disclose static magnetic field-shielding steel plates which can be adopted in color image tubes of, for example, TV monitors, with the aim of preventing color modulation on the monitors. Such steel plates are used to take advantage of their coercive force and permeability under static magnetic fields such as earth magnetic field, but cannot cope with time-varying magnetic and electric fields. Accordingly, the conventional steel plates are somewhat different from electromagnetic wave shields.
As occasion demands, construction materials are required to not permit the permeation of electromagnetic waves thereto. In this regard, hot-rolled thick plates using silicon steel are suggested for use in electromagnetic field shield constructions, as disclosed in Japanese Pat. Laid-Open Publication Nos. 2001-107201 and 2001-107202. The construction materials, however, take advantage only of the high permeability of silicon steel under static magnetic fields, and are not described in terms of electric fields. Further, the steel plates are poor in mechanical formability and platability(the property of galvanized coating) because they are not cold-rolled but hot-rolled.
Also, the present inventors disclosed a steel material with excellent magnetic shield effect at low frequencies in Korean Pat. Appl'n No. 1999-52018. The shield effect is a theoretical value obtained from the permeability and conductivity measured under static magnetic fields, and thus differs from real values, finding difficulty in practical application. Thus, there remained a need for shield evaluation under time-varying magnetic fields.
Meeting this need, methods for evaluating the magnetic shielding effect of steel plates according to frequencies were developed (Korean Pat. Nos. 2000-79907 and 2000-80886), and are in current use.
Typically, the shielding efficiency of a steel plate can be obtained by the following equations:                               Magnetic          ⁢                                           ⁢          Shield          ⁢                                           ⁢          Efficiency                =                                                                                                                        Applied                      ⁢                                                                                           ⁢                      Magnetic                      ⁢                                                                                           ⁢                      Field                                        -                                                                                                                    Transmitted                    ⁢                                                                                   ⁢                    Magnetic                    ⁢                                                                                   ⁢                    Field                                                                                      Applied              ⁢                                                           ⁢              Magnetic              ⁢                                                           ⁢              Field                                ×          100                                    Equation        ⁢                                   ⁢        1                                          Electric          ⁢                                           ⁢          Shield          ⁢                                           ⁢          Efficiency                =                                                                                                                        Applied                      ⁢                                                                                           ⁢                      Electric                      ⁢                                                                                           ⁢                      Field                                        -                                                                                                                    Transmitted                    ⁢                                                                                   ⁢                    Electric                    ⁢                                                                                   ⁢                    Field                                                                                      Applied              ⁢                                                           ⁢              Electric              ⁢                                                           ⁢              Field                                ×          100                                    Equation        ⁢                                   ⁢        2            
Expressed as dB units, the shield effect of a steel plate can be obtained by the following equations:                               Magnetic          ⁢                                           ⁢          Shield          ⁢                                           ⁢          Effect                =                              -            20                    ⁢                                           ⁢          log          ⁢                                           ⁢                                    Transmitted              ⁢                                                           ⁢              Magnetic              ⁢                                                           ⁢              Field                                      Applied              ⁢                                                           ⁢              Magnetic              ⁢                                                           ⁢              Field                                                          Equation        ⁢                                   ⁢        3                                          Electric          ⁢                                           ⁢          Shield          ⁢                                           ⁢          Effect                =                              -            20                    ⁢                                           ⁢          log          ⁢                                           ⁢                                    Transmitted              ⁢                                                           ⁢              Electric              ⁢                                                           ⁢              Field                                      Applied              ⁢                                                           ⁢              Electric              ⁢                                                           ⁢              Field                                                          Equation        ⁢                                   ⁢        4            
According to the equations, the shield effect of a shielding material having a shielding efficiency of 90% (attenuation of electromagnetic waves to one tenth) can be expressed as 20 dB. A shielding efficiency of 95% (attenuation of electromagnetic waves to one twentieth) corresponds to a shield effect of about 26 dB.
Korean Pat. Appl'n No. 2000-81056 to the present inventors is directed to a biowave steel plate based on an electromagnetic shielding cold-rolled steel plate on which powders emitting far-infrared radiation are coated. To improve the shielding effect against time-varying magnetic fields, that is, to obtain high permeability under time-varying magnetic fields, the biowave steel plate for shielding electromagnetic waves contains carbon in an amount of 0.02% or less and Si in an amount of 0.5-3.5%.
As for cold-rolled steel plates with a carbon content less than 0.02%, they are unsuitable for use in construction owing to their poor strength. Lower carbon contents in the steel plate make the grains of the steel microstructure coarser, improving the magnetic shield effect, but lowering the strength. Therefore, cold-rolled steel plates with low carbon contents are not suitable for use where high strength is required.
Also, the silicon steel plates are too high in strength and very poor in mechanical formability (elongation 40% or less), so that they are very difficult to apply to construction and household appliances which require mechanical formability of materials.
For use in exterior environments, like construction exterior finishes, silicon steel plates must be corrosion-resistant. In this regard, hot-dip galvanizing with anti-corrosive materials is carried out on such exterior finishes. However, the presence of Si is apt to cause plating defects, such as uncoating, upon the hot-dip galvanization of the steel plate. In fact, electromagnetic shield steel plates applied to highly corrosive environments, like construction finishes, are required to be hot-dip galvanized with zinc at a coating density of at least 100 g/mm2.